Arrangement in a mobile communications system for extending the range between one or more mobile units and a base station

ABSTRACT

A mobile radio system includes a number of base stations operating with time frames and mobile units operating with low transmitting power. The range between an affected mobile unit and an affected normal base station is extended by means of a repeating function. The repeating function can be effected from a functionally subordinated base station which operates with a short range. The functionally subordinated base station is placed between the normal base station and the mobile units. Each subordinated base station is arranged to detect affected mobile units which call the normal base station and to forward the calls and the information which the affected mobile units transmit to the normal base station.

TECHNICAL FIELD

The present invention relates to an arrangement for producing animproved/extended range between the mobile units and the base station ina mobile digital communication system which includes base stationsoperating with time frames and mobile units operating with lowtransmitting power.

PRIOR ART

In digital radio systems which operate with time frames and are used inthe general mobile telephone network, the coverage between a mobile unitincluded in a system and a normal base station is limited to thetransmission power/output power of the mobile unit. When the mobile unitis located within the coverage area of the normal base station affectedand transmits in uplink, it is not certain that the transmittingpower/output power of the mobile unit will be sufficient forestablishing a connection to the normal base station. In general publicrelay systems, it is previously known, from Patent SpecificationEPA-2092237, to use a repeating function for improving the effectivecoverage area. The repeating function repeats a first sequence ofdescending time slots in a first time frame from the first to the secondstation as a second sequence of descending time slots in a second timeframe. Furthermore, the repeating function sends a first sequence ofascending time slots to the first station which responds to a secondsequence of ascending time slots received from the second station. Incellular mobile telephone systems, it is previously known, from U.S.Pat. No. 4,972,456, to increase the coverage area by utilizing a numberof satellite cells, the object of which is to transfer signals betweenthe base station and the user who is located in one of the saidsatellite cells. Communication between the satellite cell and the basestation occurs wirelessly via radio communication. The communicationwhich takes place in both uplink and downlink is consequently carriedout via the satellite cell. In mobile radio networks intended for datacommunication, it is previously known, from U.S. Pat. No. 4,882,765, toutilise a repeater. The repeater listens to the communication betweenmobile unit and base station and decides itself if a message needs to berepeated or not. The transmission time does not therefore need to betaken up by unnecessary repetitions. The communication is repeated bythe repeater both in uplink and downlink.

In connection with digital mobile radio systems belonging to thiscategory and having limited range for the mobile units, a number ofmethods have been proposed for improving the coverage between the mobileunits and the base station by means of a repeating function, amongothers. The existing proposals, however, have entailed repeatingfunctions with, among others, high power consumption and fixedplacements within the coverage area of the normal base station. No knowntechnique for improving the range between the mobile units and the basestation in general mobile telephone networks (for example the GSMnetwork) which entails minimum power consumption in the repeatingfunction when no call is in progress, fast transfer of random accessinformation to the normal base station, little time delay for speechduring a call and a call channel with high transmission capacity haspreviously been found.

DESCRIPTION OF THE INVENTION TECHNICAL PROBLEM

Current modern digital mobile telecommunication systems presuppose thewidest possible effective coverage between a normal base station andmobile units in the network at the same time as being able to give thegreatest possible consideration to the low output power thathand-portable mobile units are arranged to operate with. Furthermore, itis desirable, in present-day mobile telecommunication systems, to beable to introduce repeating units/functions, which repeatingunits/functions should be possible to place optionally within thecoverage area of the normal base station without access to power orfixed connections from the remaining network. It should be possible toincrease the coverage areas considerably at locations with lowpopulation density and little development of the infrastructure. Therepeating unit/function should be able to compensate for the limitedrange in uplink, that is to say from the mobile unit/mobile units to thenormal base station, of the hand-portable mobile units. The invention isintended to be able to solve the above problems, among others.

In one embodiment of the subject-matter of the invention, a base stationfunctionally subordinated to the normal base station is utilised, whichsubordinated base station should be able to operate with low power andbe fed with power at locations with low population density and littledevelopment of the infrastructure. Furthermore, the base stationfunctionally subordinated to the normal base station should be possiblewithin an optional area which consists of the coverage area of thenormal base station. The invention is intended to be able to solve theabove problems.

In a further embodiment of the subject matter of the invention, themobile communication system is arranged with a selected link connectionby means of which selected link connection it should be possible tohandle the traffic at the same time in one or more base stationsfunctionally subordinated to the normal base station. The invention isalso intended to solve this problem.

In a further embodiment, each mobile unit is arranged to effect callsdirectly to the normal base station. It should be possible to effectthese calls (in uplink) in such a manner that the base stationsfunctionally subordinated to the normal base station are utilized aseffectively as possible. The invention is also intended to solve thisproblem. In a further embodiment of the concept of the invention, basestations functionally subordinated to a respective normal base stationare each arranged to operate with a low power state, which low powerstate should be able to take into consideration the traffic intensitywithin the affected base station functionally subordinated to the normalbase station. The invention is also intended to solve this problem.

In a further embodiment of the subject-matter of the invention, the timeslots of the base stations functionally subordinated to the normal basestation are displaced in time, which time displacement should be able tocompensate for delays within the mobile communication system.Furthermore, each base station functionally subordinated to the normalbase station is arranged with an element for detecting calls andacknowledgements. This element should be able to decide about effectinga call via the said subordinated base station. Each base stationfunctionally subordinated to the normal base station is furthermorearranged to measure signal strength. This signal strength should be ableto form the basis for effecting and transferring the connectedconnections of the mobile unit. The invention is intended to solve theproblems specified above.

SOLUTION

That which mainly can be considered to be characterizing of anarrangement according to the invention is that a repeating function iseffected by means of a base station functionally subordinated to thenormal base station. This functionally subordinated base stationoperates with short range and can be placed optionally between thenormal base station and the mobile unit/mobile units. Furthercharacterizing features are that each base station subordinated to thenormal base station is arranged to sense or detect affected mobile unitswhich make a call to the normal base station. The base stationssubordinated to the normal base station are furthermore arranged tocarry out the repeating function and forward the calls and theinformation/the messages which are transmitted by the affected mobileunit(s) to each affected normal base station.

In one embodiment of the concept of the invention, the base stationsfunctionally subordinated to the normal base station are power-fed bysolar cells, preferably using solar cells with battery backup.Furthermore, the base stations functionally subordinated to the normalbase station are optionally placeable within the coverage area of theaffected base station.

In a further embodiment of the concept of the invention, thecommunications system is arranged with a synchronization function foreach subordinated base station. The synchronization function is effectedby means of a link connection which is established for this purpose.

In a further embodiment of the concept of the invention, each of thebase stations subordinated to the normal base station is arranged onlyto forward calls when the affected mobile units cannot directly reachthe normal base station. Each of the base stations functionallysubordinated to the normal base station is also arranged to operate witha low-power state. Each subordinated base station changes to thelow-power state when no traffic is being effected via the said affectedsubordinated base station. A base station functionally subordinated tothe normal base station which has assumed a low-power state has a powerconsumption which is less than 100 mW.

In a further embodiment of the concept of the invention, the time slotsof the base stations functionally subordinated to the normal basestation are displaced in time in relation to those of the normal basestation in order to compensate for the transit delay. Each functionallysubordinated base station is arranged to effect traffic with a capacityof 16 kbit/s per established call channel.

In a further embodiment, each base station functionally subordinated tothe normal base station is provided with elements for detecting. Theelements for detecting detect calls and acknowledgement of calls fromaffected mobile units when no connection is established via the saidfunctionally subordinated base station. Furthermore, each base stationfunctionally subordinated to the normal base station is arranged tomeasure signal strength from affected mobile units. The affected mobileunits are connected to surrounding functionally subordinated basestations. The communication system transfers the connection of anaffected mobile unit to that base station functionally subordinated tothe normal base station which exhibits the highest signal strengthvalue.

ADVANTAGES

By means of that which has been proposed above, an effective mobilecommunication system can be set up in which the effective coverage areafor mobile units and normal base stations can be greatly increased. Thistype of increased coverage area is especially suitable for being used inlocations with low population density and little development ofinfrastructure (power, connections). Such areas are mainly found inmountainous regions, archipelagos and other sparsely built-up areas. Thesystem is also suitable for emergency telephones in archipelagos and inmountainous regions. The base stations subordinated to the normal basestation can be fed with power by solar cells and possibly batterybackup, providing that the call intensity is not too high. The result isthat the subordinated base stations can be located optionally, withoutaccess to power or fixed connections, within the coverage area of thenormal base station. The arrangement is particularly suitable for GSMand this type of TDMA system which creates the possibility of a very lowenergy consumption, in the subordinated base stations, during calls aswell, due to the discontinuous transmission.

DESCRIPTION OF THE FIGURE

A presently proposed embodiment of an arrangement which exhibits thecharacteristics significant of the invention will be described belowwith simultaneous reference to the attached drawings, where FIG. 1 showsa preferred embodiment of the invention in block diagram form.

FUNCTIONAL EMBODIMENT

FIG. 1 shows a mobile digital telecommunication system comprising partlya number of base stations operating with time frames, one is designatedas 1 here, and partly a number of mobile units, two are designated as 2and 3 here. The base station 1 is connected by means of a connection 4to a mobile telephone exchange 5. The mobile telephone exchange 5constitutes the interface to the normal telephone network 6 and isconnected to a switching station 7. The mobile communication system canconsist of a system known per se, the mobile radio systems with thedesignations NMT and GSM can be named as examples. In the figure, acoverage area belonging to the base station 1 is designated by 8. Thesize of the coverage area 8 for a base station 1 in the mobiletelecommunication system is limited by the output power which the mobileunits 2, 3 are arranged to operate with. Where the mobile units 2, 3consist of hand-portable mobile telephones, their output power is ratherlow. Within the coverage area of the base station 1 are placed a numberof base stations subordinated to the normal base station 1 heredesignated by 9, 10, 11. To extend the range, that is to say to increasethe coverage area 8 for the normal base station 1, the base stations 9,10, 11 subordinated to the normal base station 1 are arranged to detectthe mobile units 2, 3 when they call the normal base station 1. Each ofthe base stations 9, 10, 11 subordinated to the normal base station 1receives messages which are sent from the mobile units 2, 3 to thenormal base station 1 and thereafter transmit the said message via aradio link 9', 10', 11' to the normal base station 1. Messages and callsfrom the normal base station 1 are sent directly to the mobile units 2,3. When a mobile unit 2 is located within the coverage area 8 of thenormal base station 1, calls and messages are sent from the mobile unit2 directly to the normal base station 1 by means of a radio link 12.

The coverage area 8 of a base station (cell) 1 in a mobile telephone islimited by the low output power from, above all, hand-portable mobiletelephones 2, 3. The coverage area can be greatly increased in, forexample, sparsly built-up areas, mountainous regions and archipelagos,by a system concept with a normal base station 1 and a network of basestations 9, 10, 11 functionally subordinated to the normal base station.These base stations 9, 10, 11 subordinated to the normal base stationcan be fed with power by solar cells 100 possibly with battery backup120, provided that the call intensity is not too high. In FIG. 1, thebase stations 9, 10, 11 subordinated to the normal base station areespecially adapted to the GSM system and this type of TDMA system, whichcreates the possibility of a very low power consumption even duringcalls, due to the discontinuous transmission. These base stations 9, 10,11 subordinated to the normal base station can therefore be optionallyplaced without access to power or fixed connections and are particularlysuitable for use in locations with low population density and littledevelopment of infrastructure (power, connections). The system is alsosuitable for network telephones in archipelagos and in mountainousregions. The system concept is based on those channels which are totransmit continuously from the network side being handled by the normalbase station which operates with high output power and therefore longrange. The normal base station 1 leaves all the remaining communicationto the base stations 9, 10, 11 subordinated to the normal base station.The base stations 9, 10, 11 subordinated to the normal base station onlyneed to listen for calls and acknowledgements of calls from the mobileunits 2, 3 when no connection is established via them. In certain systemapplications, the base stations 9, 10, 11 subordinated to the normalbase station 1 must be accurately synchronized with the normal basestation 1. Moreover, the calls must be forwarded to them. For thispurpose, a special link connection 9', 10', 11' is set up. During a callin the downlink, that is to say from the normal base station 1 to themobile units 2, 3, a direct connection from the normal base station tothe mobile units is used. For the uplink, that is to say from the mobileunits to the normal base stations, the base stations 9, 10, 11subordinated to the normal base station are intended to forward the call(when the mobile unit 3 does not reach the normal base station 1directly).

To retain full GSM functionality for the mobile unit 2, 3 connected tothe GSM, a number of GSM functions must be modified on the network side.The result of this is that, for the surrounding part of the GSM network,the normal base station 1 including the base stations 9, 10, 11subordinated to the normal base station look like a single normal cell(with standard GSM interface). The extra functions and interfacesrequired only affect the normal base station and the base stations 9,10, 11 functionally subordinated to the normal base station.

The power consumption in the base stations 9, 10, 11 subordinated to thenormal base station is much lower than 100 mW when no call is inprogress. To achieve this, only the system parts necessary at each timepoint are provided with current supply. Solar cells in combination withchargeable accumulators can be used as current source.

The mobile units 2, 3 are arranged only to measure the signal strengthtowards the normal base station and its normal neighbours during thecall. To be able to carry out transfer/hand-over between the basestations 9, 10, 11 functionally subordinated to the normal base station,the surrounding base stations 9, 10, 11 functionally subordinated to thenormal base station are arranged to measure the signal strength towardsthe mobile unit 2, 3. This measurement only needs to be taken when thequality and/or signal strength of the call becomes too low and dropsbelow a predetermined limit value. It can even be conceived that thecall in the uplink be handed over to another base station 9, 10, 11functionally subordinated to the normal base station by this basestation beginning to receive on the same frequency and in the same timeslot, a so-called soft hand-over.

The invention is not limited to the embodiment shown above as an examplebut can undergo modifications within the scope of the subsequent patentclaims and concept of the invention.

I claim:
 1. A subordinate base station, which is functionallysubordinate to a main base station in a mobile communications systemthat includes a mobile station, said subordinate base station and a mainbase station, wherein said main base station is configured to operatebased on time frames, and said mobile station configured to operate witha lower transmit power than said main base station, said subordinatebase station being located between said main base station and saidmobile station, said subordinate base station comprising:a transmitterconfigured to transmit signals over a shorter range than said main basestation; a sensor configured to detect a signal level of a signal sentfrom said mobile station to said main base station; and a repeatingmechanism configured to forward said signal from said mobile station tosaid base station when said sensor detects that said signal level isless than a predetermined level, wherein said repeating mechanismoperates on an uplink from said mobile unit to said main base station,but not on said downlink.
 2. The subordinate base station of claim 1,further comprising a solar cell having a battery backup.
 3. Thesubordinate base station of claim 1, wherein said subordinate basestation is located within a coverage range of said main base station. 4.The subordinate base station of claim 1, further comprising a selectedlink connection synchronizing mechanism configured to synchronize anarrival of said signal forwarded by said subordinate base station withsignals forwarded by other subordinate base stations.
 5. The subordinatebase station of claim 1, configured to allow said mobile unit totransmit said signal directly to said main base station when said mobileunit is located within said coverage area of said main base station. 6.The subordinate base station of claim 1, wherein said subordinate basestation is configured to operate in a low power operational mode thatdissipates less than 100 mW when no signal is being forwarded thereby.7. The subordinate base station of claim 1, wherein said subordinatebase station is configured to compensate for transit delays bydisplacing time frames used for forwarding the signal with respect tothe time frames established at the main base station.
 8. The subordinatebase station of claim 1, wherein said signal is forwarded at a rate of16 kbp/s per established call channel.
 9. The subordinate base stationof claim 1, further comprising:a detection and acknowledgment mechanismconfigured to detect a call and acknowledge a call from the mobile unitwhen the subordinate base station does not connect the mobile station tothe base station.
 10. The subordinate base station of claim 1, whereinsaid sensor and said repeating mechanism are cooperatively configured tomeasure respective signal strengths of signals from other mobilestations connected to other subordinate base stations or the subordinatebase station, and effect a hand-over operation to a subordinate basestations that exhibits a highest signal strength value.
 11. Thesubordinate base station of claim 2, wherein said subordinate basestation is located within a coverage range of said main base station.12. The subordinate base station of claim 2, further comprising aselected link connection synchronizing mechanism configured tosynchronize an arrival of said signal forwarded by subordinate basestation with signals forwarded by other subordinate base stations. 13.The subordinate base station of claim 3, further comprising a selectedlink connection synchronizing mechanism configured to synchronize anarrival of said signal forwarded by subordinate base station withsignals forwarded by other subordinate base stations.
 14. Thesubordinate base station of claim 11, further comprising a selected linkconnection synchronizing mechanism configured to synchronize an arrivalof said signal forwarded by said subordinate base station with signalsforwarded by other subordinate base stations.
 15. The subordinate basestation of claim 2, configured to allow said mobile unit to transmitsaid signal directly to said main base station when said mobile unit islocated within said coverage area of said main base station.
 16. Thesubordinate base station of claim 2, wherein said subordinate basestation is configured to operate in a low power operational mode thatdissipates less than 100 mW when no signal is being forwarded thereby.17. The subordinate base station of claim 2, wherein said subordinatebase station is configured to compensate for transit delays bydisplacing time frames used for forwarding the signal with respect tothe time frames established at the main base station.
 18. Thesubordinate base station of claim 2, wherein said signal is forwarded ata rate of 16 kbp/s per established call channel.
 19. The subordinatebase station of claim 2, further comprising:a detection andacknowledgment mechanism configured to detect a call and acknowledge acall from the mobile unit when the subordinate base station does notconnect the mobile station to the base station.
 20. The subordinate basestation of claim 2, wherein said sensor and said repeating mechanism arecooperatively configured to measure respective signal strengths ofsignals from other mobile stations connected to other subordinate basestations or the subordinate base station, and effect a hand-overoperation.